Technical Field
The present invention relates to electrostatic discharge devices, and more particularly to structures and methods for protecting semiconductor devices with discharge to a bulk substrate of a wafer.
Description of the Related Art
Electrostatic discharge (ESD) protection may be afforded in semiconductor devices by dumping charge into large power supply buses. This charge dump dissipates energy in large capacitances provided by the large power supply buses. The ESD dump is sometimes accomplished by a pair of up- and down-diodes, where the up-diode directs positive pulses into a supply voltage bus (Vdd or VDD) and is normally off. The down-diode directs negative-going pulses into a ground bus, and is also normally off.
In conventional semiconductor-on-insulator (SOI) technology standard diodes may be used. However, in extremely thin SOI (ETSOI) substrates, it is advantageous to use devices built in bulk silicon for improved power dissipation rather than the extremely thin semiconductor layer. In some manifestations of ESD circuits, a bias voltage may be applied to a bulk wafer instead of chip ground, and in that case, the negative-going pulse should not be dumped into the bulk. In such a case, the bulk wafer is not chip ground, but is instead isolated from chip ground by a pump circuit.